The present invention relates to method and apparatus for determining in situ the density and fabric or structure of soils, sands, silts, and other materials. More particularly, the present invention provides a method and implementing apparatus for measuring in situ the electrical capacitance and resistance of soil samples at a predetermined depth and determining therefrom the density and fabric or structure of the soil. Other uses are also indicated.
A knowledge of the in situ density and structure of sands, loams, and silts is necessary for the accurate prediction of the settlement and liquefaction potentials of these soils. A recognition of the danger of these soils behaving as a liquid in earthquakes has brought about a practical requirement that the liquefaction potential of all natural deposits and fills of soils be evaluated for all construction projects.
It has been generally understood that the behavior of granular media, such as soils, sands, loams, silt, and other material, depends mainly on overall structure rather than the properties of each individual grain. With sands, for example, an average size range of the grains may be determined; whereupon the term "structure" (sometimes called "fabric") is related to three basic features: the shape of the solid particles, the spatial arrangement of the solid particles, and the associated voids. While it has been recognized that these three basic features must be taken into account in any index used to quantify sand structure, the practical results have not been satisfactory.
Porosity alone, for example, is known to be inadequate. Relative density has proven a better index than porosity, but has dual drawbacks found (1) in difficulty of evaluating its value accurately and (2) in variation of properties resulting from different methods of preparation of sand samples.
A common disadvantage shared by the indices of, e.g., porosity and relative density is that they are scalar quantities not associated with any particular direction in the sample. Hence, they cannot possibly account for the anisotropic character of the sand mass resulting from the shapes and spatial arrangements of the solid particles.
Electrical methods have been used in the past to determine the density of sands. H. Camberfort, in his paper entitled "In Situ Measurement of the Porosity of Sand," published in the Proceedings of the 4th International Conference on Soil Mechanics, London, 1957, at pages 213-215, sets forth an empirical equation which related the void ratio (e) of sand to the resistivity of the sand-solution mixture (P), and the resistivity of the pore fluid (P.sub.0), that is, the fluid in the interstices or pores of the soil. That empirical equation was: ##EQU1## where P/P.sub.0 was defined as the "Formation Factor", F. According to Camberfort's equation (Equation 1), the Formation Factor (F) was only a function of the void ratio or density of sands.
It has also been shown by Fraser and Ward in their paper, "Electrical Pore Space Geometry of Porous Media," Report No. MT-63-8, July 1963, Institute of Engineering Research, Berkeley, Calif. that: EQU F=(t.sup.2 /n) (2)
where n is the porosity and t is the tortuosity coefficient, defined as the ratio of the electrolyte path over the straight path along the direction of the measurement. In other words, the tortuosity coefficient (t) was shown to be a measure of the contortion of the path of ionic conduction due to the sand structure. Thus, the tortuosity coefficient will differ according to the shapes of the solid particles and the angle of the direction of measurement relative to the spatial arrangement of the solid particles.
G. Y. Chernyak, in his 1967 paper entitled "Dielectric Methods for Investigating Moist Soils," (translated from the Russian by N. Kaner, U.S. Department of Commerce, TT67-51258) empirically related the porosity (n) of sands to the dielectric constant (E') of the sand-solution mixture, the dielectric constant (Er) of the sand particles, and the dielectric constant (Es) of the pure fluid, in the equation: ##EQU2##
Mulilis, Seed, Chan, Mitchell, and the present inventor, in their paper entitled "Effect of Method of Sample Preparation", published in the Journal of the Geo-Technical Engineering Division, American Society of Civil Engineers, in February 1977, demonstrated that the fabric of sand was dictated by the method of sample preparation. Therein, soil fabric was determined by a microscopic study of thin sections obtained from a resin-impregnated sample of sand.